when you change the format of cell(s) to cardinally different one, like number to text or other way around, the actual values entered before dont get new formats automatically. to change the format of those entries you have.

a) edit the entry, i.e. select a cell and press F2, and then press Enter, or double-click on cell, and then press Enter;
or
b) make some operations on cell(s) with changed format. p.e. to format previous texts to numbers you copy number 1 into some empty cell, copy the cell, select the range where you changed formats and then use PasteSpecial Multiply operation on this range. Or to format previous numbers to text you enter into some free cell the formula = "" & CellWithNumber, copy it to range so all numbers are converted, and then use PasteSpecial Values operation to overwrite numbers in range with changed format with calculated ones.

When you change p.e. numbers to dates by merely formatting, this is not needed, because really dates are same numbers - only displayed in different way.